Conventionally, with regard to such an internal combustion engine variable compression ratio system, there is a known system (1) in which a piston outer is screwed around the outer periphery of a piston inner, and rotating the piston outer forward and backward so that it approaches and recedes from the piston inner moves it to a low compression ratio position and a high compression ratio position (for example, Japanese Patent Application Laid-open No. 11-117779), and a known system (2) in which a piston outer is fitted in an axially slidable manner around the outer periphery of a piston inner, an upper hydraulic chamber and a lower hydraulic chamber are formed between the piston inner and the piston outer, and supplying hydraulic pressure alternately to these hydraulic chambers moves the piston outer to a low compression ratio position and a high compression ratio position (for example, Japanese Patent Publication No. 7-113330).
However, in the above-mentioned system (1), since it is necessary to rotate the piston outer in order to move it to the low compression ratio position and the high compression ratio position, the shape of the top face of the piston outer cannot be set freely so as to match the shape of the roof of a combustion chamber and the positional arrangement of intake and exhaust valves, and it is difficult to increase the compression ratio of the engine sufficiently at the high compression ratio position. Furthermore, in the above-mentioned system (2), particularly when the piston outer is at the high compression ratio position, since a large thrust load imposed on the piston outer during an expansion stroke of the engine is borne by the hydraulic pressure of the upper hydraulic chamber, it is necessary for the upper hydraulic chamber to have a seal that can withstand high pressure and, moreover, if bubbles are generated in the upper hydraulic chamber, since the high compression ratio position of the piston outer becomes unstable, it is necessary to provide means for removing such bubbles, thus inevitably increasing the overall cost.